This goal of this project is to characterize early-life effects on adult levels of physical activity, using a novel, polygenic mouse model. Selectively bred High Runner and non-selected Control lines of mice will be subjected to early-life exercise and/or Western diet (high in fat and sugar). Three separate experiments will apply these treatments to (1) mothers from before conception to parturition (prenatal period), (2) mothers from parturition until their offspring begin to eat independently (early postnatal period), and (3) offspring from weaning until sexual maturation (juvenile period). Traits of their adult male offspring will then be characterized, including voluntary exercise on wheels, spontaneous physical activity in the home cage, food consumption, body composition, basal metabolic rate, maximal aerobic capacity, dietary preferences, and other physiological and behavioral traits related to physical activity. Results will provide novel insights not only into effects of early-lie environment on adult activity levels and related measures, but also into interactions of these effects with genetic background. Relevance Results of this research will provide novel information on how adult levels of physical activity may be altered by diet and exercise during early life (prenatal, early postnatal, and juvenile periods). Findings from this work could lead to development of behavioral or dietary interventions that could be translated into human clinical interventions to prevent such inactivity-related health problems as obesity, the metabolic syndrome, Type 2 diabetes, cardiovascular diseases, and osteoarthritis.